Liquid crystal display panel capable of reducing persistence degree and development method thereof

ABSTRACT

AC voltage of rectangular wave is applied between a pixel electrode  25 A and a common electrode  23 A, and the amplitude Vac of the AC voltage component and the DC voltage component Vdc thereof are changed to measure the range of optimal DC component variation ΔVdc and determine a structure or material of a liquid crystal display device so as to lower ΔVdc less than a given value, wherein ΔVdc=|Vdcb−Vdcw|, Vdcb is the value of Vdc at which the range of transmittance variation is the minimum with Vac being fixed at a value for displaying black (2V), and Vdcw is the value of Vdc at which the range of transmittance variation is the minimum with Vac being fixed at a value for displaying white (7 V). Thickness of an insulating layer  26 A on the pixel electrode  25 A and on the common electrode  23 A are the same. Electrode crossover portions are made to be in axial symmetry. The top surface of each stripe electrode of a pixel electrode has convex shape in cross section.

This is a divisional of application Ser. No. 11/312,912, filed Dec. 20,2005, which is a divisional of application Ser. No. 10/747,517, filedDec. 29, 2003, now U.S. Pat. No. 7,095,473, which prior application wasa divisional of application Ser. No. 09/927,005, now U.S. Pat. No.6,819,384.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display (LCD) panelcapable of reducing a persistence degree and a development methodthereof.

2. Description of the Related Art

Each of FIGS. 31 and 32 is a schematic sectional view showing astructure of one pixel of an LC panel. FIG. 31 shows a state where novoltage is applied, and FIG. 32 shows a state where a voltage isapplied.

The LCD panel includes substrates 10 and 20 opposing to each other, anda sealed-in nematic liquid crystal 30 having an anisotropic dielectricpositive constant. In the substrate 10, a flat electrode 12, adielectric layer 13 and a vertically oriented layer 14 are formed on aface of a transparent insulating substrate 11, for example, a glasssubstrate, and on the other face thereof, a polarizer 15 is formed. Inthe substrate 20, a common electrode 23 is formed on one face of atransparent substrate 21, for example, a glass substrate, an insulatinglayer 24 is formed thereon, a pixel electrode 25 is formed on theinsulating layer 24, and further an insulating layer 26 and a verticallyorientated later 27 are formed thereon. On the other face of thesubstrate 21, a polarizer 28 is formed. Transmission axes of thepolarizers 15 and 28 perpendicularly cross over each other.

When backlight in the direction shown by arrows in FIG. 31 enters intothe LCD panel, the light is transformed into linearly polarized light bythe polarizer 28. When the flat electrode 12, the common electrodes 23and the pixel electrode have the same potential, the liquid crystal 30effects no change in the plane of polarization of the linearly polarizedlight, and therefore the linearly polarized light cannot be transmittedthrough the polarizer 15, resulting in a dark state.

When, as shown in FIG. 32, the flat electrode 12 and the commonelectrode 23 has the same potential but the pixel electrode 25 isapplied with a potential different from the both former electrodes, anelectric field arises. Dotted lines of FIG. 32 show the lines ofelectric force. Liquid crystal molecules are inclined relative to anincident light direction under influence of the electric field to causebirefringence, and part of the light can transmit through the polarizer15, resulting in a bright state.

Since the common electrode 23 and the pixel electrode 25 are made of anopaque metal, behaviors of liquid crystal molecules over the electrodesare not problematic in terms of display.

If the flat electrode 12 does not exist, liquid crystal moleculesbetween the pixel electrode and the common electrode 23 tend to reduceinclination thereof, which will produces the drop region oftransmittance. The flat electrode 12 makes the electric field betweenthe common electrode 23 and the pixel electrode 25 asymmetric so as tocontributes to prevent the transmittance from locally dropping. Thedielectric layer 13 reinforces the lateral component of the electricfield in the liquid crystal 30 to make it possible for the liquidcrystal 30 to be driven with lower applied voltage. The common electrode23 and the pixel electrode 25 each are stripe electrodes extending inthe direction perpendicular to the sheet of FIG. 32, and alternatelyformed on the top and bottom surfaces of the insulting layer 24. Theinsulating layer 24 is for preventing common electrodes and pixelelectrodes from short-circuiting at positions where the both overlap aswill be described later. The insulating layer 26 is for reducing thepersistence degree.

FIG. 33 shows an electrode pattern of one pixel, formed in the substrate20 of FIG. 31. FIGS. 34 and 35 are patterns of the pixel electrode 25and the common electrode 23, respectively, of FIG. 33.

In FIG. 33, a data line DL1 and a scan line SL1 cross over each otherwith an insulating layer interposing therebetween. Each of the pixelelectrode 25 and the common electrode 23 has a stripe section and aperipheral section connecting ends of the stripe section. The lines ofthe stripe section are inclined 45 degrees to each of the scan line SLland the data line DL1.

When the potential of the scan line SLl goes high, a TFT 29 is turned onto apply the potential of the data line DL1 onto the pixel electrode 25and generate an electric field between the stripe electrodes of thepixel electrode 25 and the common electrode 23. The longitudinaldirection of the upper half of the stripe electrodes is different fromthat of the lower half of the stripe electrodes by 90 degrees as shownin FIG. 33, whereby the LCD panel has wider range of viewing angles thanin a case where the both halves of the stripe electrodes are parallel toeach other.

The common electrode 23A has peripheral protrusions which are connectedto the common electrodes of adjacent pixels not shown.

FIG. 36(A) is an enlarged partial view near a crossover of a stripeelectrode and the peripheral section of FIG. 33. FIG. 36(B) shows thelines of electric force with dotted lines near the crossover when avoltage is applied between the pixel electrode 25 and the commonelectrode 23.

A peripheral section of the pixel or common electrode has crossoverportions to stripe electrodes of the common or pixel electrodes with theinsulating layer interposing therebetween since a pixel has arectangular shape, and each of the pixel electrode 25 and the commonelectrode 23 has stripe electrodes in parallel to each other and has acontinuous shape. For example, a side 251 of the pixel electrode 25 isconnected to a side 252 of the peripheral section, and a side 231 of thecommon electrode 23 is parallel to the side 251, while the side 231crosses over the side 252 at an acute angle.

FIG. 37 is a schematic sectional view showing inclination of liquidcrystal molecules between the pixel electrode 25 and the commonelectrode 23 of one pixel of the LCD panel when a voltage is appliedtherebetween.

In FIG. 32, a structure between the pixel electrode 25 and the liquidcrystal 30 is different from that between the common electrode 23 andthe liquid crystal 30, which causes persistence.

In FIG. 36(B), since the side 252 crosses over the side 231 at an acuteangle, an electric field therebetween near the crossover is strongerthan that between the parallel sides. Further, a direction of electricfield strength near the crossover is different from that between theparallel sides. Due to such conditions, a transmittance-voltagecharacteristic near the crossover is different from that between theparallel portion, causing not only degradation of an image quality butalso persistence.

In FIG. 37, since the insulating layer 26 exists above the pixelelectrode 25, application of an electric field in this portion isuseless and effective application of the electric field to the liquidcrystal 30 is prevented. If the insulating layer 26 is omitted in orderto solve this problem, it causes more persistence since the insulatingresistance of the vertically oriented layer 27 is low. If the pixelelectrode 25 is exposed to the liquid crystal 30, not only is the degreeof persistence enhanced, but liquid crystal molecules also decompose.Further, since the top surface of a pixel electrode 25 is flat, it isnot possible to effectively apply an electric field to the liquidcrystal 30 in relation to transmittance, which prevents achieving highercontrast display.

In development of an LCD panel, measurement of a persistence degree isperformed at each trial when a structure or material of the LCD panel ischanged in order to reduce the persistence degree to a value lower thana given value, and it takes, for example, 48 hours to measure thepersistence degree in each trial, which makes a development term thereoflonger.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide aliquid crystal display panel capable of reducing a persistence degree.

In one aspect of the present invention, there is provided a liquidcrystal panel comprising: first and second substrate; and liquid crystalinterposed between the first and second substrates; the first substratecomprising: an insulating substrate; first and second electrodes, formedover the insulating substrate, for a display voltage to be appliedtherebetween; and a first insulating layer covering the first and secondelectrodes; wherein the first electrode is disposed higher than thesecond electrode in relation to a direction from the insulatingsubstrate toward the second substrate, and the first and secondelectrodes overlap each other with a second insulating layer beinginterposed therebetween at an overlapping portion, wherein a thicknessof the first insulating layer on the first electrode is substantiallyequal to the insulating layer on the second electrode.

With this configuration, when the voltage signal is applied between thefirst and second electrodes, electric states over the first and secondelectrodes are almost the same, whereby persistence is reduced incomparison with a case where the thicknesses are different from eachother as shown in FIG. 31

Other aspects, objects, and the advantages of the present invention willbecome apparent from the following detailed description taken inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram of a liquid crystal display devicefor use in a method of the present invention.

FIG. 2 is an illustration of a persistence degree.

FIG. 3 is a graph showing a voltage waveform, having an AC amplitude Vacand a DC component Vdc, applied on a liquid crystal pixel.

FIG. 4 are graphs showing a measured transmittance waveform of theliquid crystal pixel in a case where the AC amplitude Vac is 2 V and theDC component Vdc is −3 V.

FIG. 5 is a graph showing a measured transmittance waveform of theliquid crystal pixel in a case where the AC amplitude Vac is 2 V and theDC component Vdc is −2V.

FIG. 6 is a graph showing a measured transmittance waveform of theliquid crystal pixel in a case where the AC amplitude Vac is 2 V and theDC component Vdc is −1V.

FIG. 7 is a graph showing a measured transmittance waveform of theliquid crystal pixel in a case where the AC amplitude Vac is 2 V and theDC component Vdc is −0.5 V.

FIG. 8 is a graph showing a measured transmittance waveform of theliquid crystal pixel in a case where the AC amplitude Vac is 2 V and theDC component Vdc is 0 V.

FIG. 9 is a graph showing a measured transmittance waveform of theliquid crystal pixel in a case where the AC amplitude Vac is 2 V and theDC component Vdc is 0.5 V.

FIG. 10 is a graph showing a measured transmittance waveform of theliquid crystal pixel in a case where the AC amplitude Vac is 2 V and theDC component Vdc is 1 V.

FIG. 11 is a graph showing a measured transmittance waveform of theliquid crystal pixel in a case where the AC amplitude Vac is 2 V and theDC component Vdc is 2 V.

FIG. 12 is a graph showing a measured transmittance waveform of theliquid crystal pixel in a case where the AC amplitude Vac is 2 V and theDC component Vdc is 3 V.

FIG. 13 is a graph showing a measured relationship between the DCcomponent Vdc and the variation width ΔT of liquid crystal pixeltransmittance in a case where the AC amplitude Vac is 2 V.

FIG. 14 is a graph showing a measured relationship between the ACamplitude Vac, and the value of DC component Vdc at which the variationwidth ΔT of liquid crystal pixel transmittance is the minimum.

FIG. 15 is a graph showing a measured relationship between a persistencedegree and the range of optimal DC component variation ΔVdc.

FIG. 16 is a schematic sectional view showing a structure of a liquidcrystal pixel capable of reducing a persistence degree in a state whereno voltage is applied, of a second embodiment according to the presentinvention.

FIG. 17 is a schematic sectional view showing the liquid crystal pixelof FIG. 16 in a state where a voltage is applied.

FIGS. 18(A)-18(F) are schematic sectional views showing a fabricationprocess of the substrate 20A of FIG. 16.

FIG. 19 is a plane view showing an electrode pattern of a liquid crystalpixel capable of reducing a persistence degree, of a third embodimentaccording to the present invention.

FIG. 20 is a plane view showing the pixel electrode of FIG. 19.

FIG. 21 is a plane view showing the common electrode of FIG. 19.

FIG. 22(A) is an enlarged partial view near a crossover of a stripeelectrode and the peripheral section of FIG. 19.

FIG. 22(B) is a diagram showing the lines of electric force with dottedlines near the crossover when a voltage is applied between theelectrodes of FIG. 22(A).

FIG. 23 is a plane view showing an electrode pattern of a liquid crystalpixel capable of reducing a persistence degree, of a fourth embodimentaccording to the present invention.

FIG. 24 is a plane view showing the common electrode of FIG. 23.

FIG. 25 is a plane view showing an electrode pattern of a liquid crystalpixel capable of reducing a persistence degree, of a fifth embodimentaccording to the present invention.

FIG. 26 is a plane view showing the common electrode of FIG. 25.

FIG. 27 is a plane view showing an electrode pattern of two liquidcrystal pixels adjacent to each other, of a sixth embodiment accordingto the present invention.

FIG. 28 is an enlarged sectional view taken along line A-A of FIG. 27.

FIGS. 29(A)-29(C) are schematic sectional views showing a fabricationprocess of a substrate on the back light incident side.

FIGS. 30(A)-30(B) are schematic sectional views showing the fabricationprocess following FIG. 29.

FIG. 31 is a schematic sectional view showing a structure of one pixelof an LCD panel compared to the present invention in a state where novoltage is applied.

FIG. 32 is a schematic sectional view showing the pixel of FIG. 31 in astate where a voltage is applied.

FIG. 33 is a plane view showing an electrode pattern of one pixel formedin the substrate 20 of FIG. 31.

FIG. 34 is a plane view showing the pixel electrode of FIG. 33.

FIG. 35 is a plane view showing the common electrode of FIG. 33.

FIG. 36(A) is an enlarged partial view of the pattern near a crossoverbetween electrodes of FIG. 33.

FIG. 36(B) is a diagram showing the lines of electric force with dottedlines when a voltage is applied between the electrodes of FIG. 36(A).

FIG. 37 is a schematic sectional view showing inclination of liquidcrystal molecules between a pixel electrode and a common electrode ofone pixel of a prior art LCD panel when a voltage is appliedtherebetween.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference charactersdesignate like or corresponding parts throughout several views,preferred embodiments of the present invention are described below.

First embodiment

First of all, there will be described a development method capable ofdecreasing the development term of the LCD panel which has a structureor employs material capable of reducing a persistence degree.

FIG. 1 is a schematic circuit diagram of a liquid crystal display devicefor use in a method according to the present invention. FIG. 1 shows acase where a pixel array has a matrix with 3 rows and 6 columns forsimplicity.

The circuit itself is the same as that of the prior art. A data lineDL1, a scan line SL1, TFT 29, a pixel electrode 25, a common electrode23, and a flat electrode 12 in FIG. 1 are formed as shown in FIG. 31 forexample. The flat electrode 12 is provided for all the pixel electrodes.The scan lines are connected to the output of a scan driver 31, and thedata lines are connected to the output of a data driver 32. A controlcircuit 33 controls the data driver 32 on the basis of a pixel clock CLKand a horizontal sync signal HSYNC, and also provides a video signal VSto the data driver 32, and further controls the scan driver 31 on thebasis of the horizontal sync signal HSYNC and a vertical sync signalVSYNC. Rows of a pixel array are line-sequentially selected by the scandriver 31, and display data (a gradation voltage set) is provided to thepixels of a selected row from the data driver 32.

FIG. 2 is an illustration of a persistence degree.

For example, a case is considered where display data of each pixel has a64-step gradation, white corresponds to the sixty-fourth gradation andblack corresponds to the first gradation. The persistence degree isdefined as follows:

(A) A fixed pattern including white and black is displayed, for example,for 48 hours.

(B) Immediately thereafter, the halftone of the thirty-second gradationis displayed, and the brightness Bmw and Bmb in respective regions inwhich the white and black was displayed in the step (A) are measured.The persistence degree PD is calculated with the following formula:PD=100(Bmw−Bmb)/Bm %

where Bm is a smaller one of the Bmw and Bmb.

In the step (B), in order that no persistence can be recognized by ahuman, a persistence degree has to be less than 6% under ordinaryillumination in a room and less than 3% in dark room.

The persistence degree is different according to a structure or materialof an LCD panel. In development of an LCD panel, if the persistencedegree is measured at each trial when a structure or material of an LCDpanel is changed in order to reduce the persistence degree to a valuelower than a given value, and it takes, for example, 48 hours to measurethe persistence degree in each trial, which makes a development termthereof longer. Therefore, it is effective to search a physical quantityhaving a high correlation with the persistence degree and which can bemeasured in a short time.

Liquid crystal pixel is applied with an AC voltage of rectangular wavein order to prevent its degradation. FIG. 3 shows a voltage waveformapplied between the pixel electrode 25 and the common electrode 23, andbetween the pixel electrode 25 and the flat electrode 12 of FIG. 32,wherein a frequency is 30 Hz.

The voltage waveform has a DC component in order to prevent flickersfrom arising under the application of the only AC voltage, that is, inorder to avoid a cyclical change in transmittance. The amplitude of theAC voltage of rectangular wave and the DC voltage component areindicated by Vac and Vdc, respectively.

The LCD panel transmittance was measured each time the DC component Vdcwas altered stepwise with the AC amplitude being fixed. FIGS. 4 to 12show variations in transmittance in cases where the DC component Vdc wasset at −3V, −2V, −1 V. −0.5V, 0 V, 0.5 V, 1 V, 2 V and 3 V,respectively, with the AC amplitude Vac being fixed at a black displayvoltage 2V. As shown in FIG. 4, the range of transmittance variation isindicated by ΔT.

FIG. 13 is a graph showing a relationship between the DC component Vdcand the range of transmittance variation ΔT in a case where the ACamplitude Vac is 2 V. It is estimated from this graph that the value ofthe DC component Vdc at which the range of transmittance variation ΔT isthe minimum is −0.38 V.

Likewise, measured are the value of the DC component Vdc at which therange of transmittance variation ΔT assumes the minimum ΔTmn in caseswhere the AC amplitude Vac is a white display voltage 7 V and a halftonedisplay voltage (2+7)/2=4.5 V. FIG. 14 shows the results obtained by themeasurement, wherein ΔVdc denotes the range of optimal DC componentvariation. The DC component Vdc is fixed in an actual liquid crystaldisplay device. Therefore, as the range of optimal DC componentvariation ΔVdc decreases, flickers become weaker.

FIG. 15 is a graph showing a relationship between the persistence degreeand the range of optimal DC component variation ΔVdc, obtained bymeasuring the persistence degree and the range of optimal DC componentvariation ΔVdc each time the structure or material of an LCD panel ischanged. It can be seen that there is a very high correlation betweenthe persistence degree and the range of optimal DC component variationΔVdc. Further, it is found that in order to lower the persistence degreeless than the above described 6%, the range of optimal DC componentvariation ΔVdc has to be less than 0.5 V, and in order to lower thepersistence degree less than the above described 3%, the range ofoptimal DC component variation ΔVdc has to be less than 0.2 V.

Since the range of optimal DC component variation ΔVdc can be measuredin a short time with ease, by use of ΔVdc it is possible to reduce thedevelopment term of an LCD panel with the persistence degree being lessthan a given value.

Note that it was confirmed that there is a high correlation between therange of optimal DC component variation ΔVdc and the persistence degreeeven in LCD panels having structures where the pixel electrode 25 andthe common electrodes 23 are employed without the flat electrode 12 andthe pixel electrode 25 and the flat electrode 12 are employed withoutthe common electrodes 23, and therefore there will be a similarcorrelation therebetween in LCD panels having other structures.

Second Embodiment

FIGS. 16 and 17 are schematic sectional views showing a structure of aliquid crystal pixel capable of reducing the persistence degree, of asecond embodiment according to the present invention, wherein FIG. 16shows a state where no voltage is applied and FIG. 17 shows a statewhere a voltage is applied.

The structure of a substrate 20A is different from that of the substrate20 of FIG. 31. The other structure is the same as that of FIG. 31.

FIG. 18 are schematic sectional views showing a fabrication process ofthe substrate 20A. In FIG. 18, the right end portions of (A) to (F)indicate a place where a common electrode 23A and a pixel electrode 25Aare stacked with an insulating layer 24A interposing therebetween.

(A) A common electrode 23A made of metal is formed on a transparentinsulating substrate 21 by photolithography.

(B) an insulating layer 24 is coated on the substrate 21.

(C) A pixel electrode 25A is formed on the insulating layer 24 byphotolithography.

(D) The insulating layer 24 is etched with the pixel electrode 25A as amask and the only portion thereof under the pixel electrode 25A is left.

(E) An insulating layer 26A is coated on the substrate 21.

(F) A vertically oriented layer 27 is coated on the insulating layer26A.

By fabricating the substrate 20A in such a way, as shown in FIG. 16, thethicknesses of the insulating layer 26A over the pixel electrode 25A issubstantially equal to that over the common electrode 23A. Therefore,electric states over and near the common electrode 23A and over and nearthe pixel electrode 25A are almost the same as shown in FIG. 17 in acase where an AC voltage of rectangular wave is applied between thepixel electrode 25A and the common electrode 23A, and the persistence isreduced in comparison with an LCD panel having the structure of FIG. 31.In other words, the range of optimal DC component variation ΔVdc of FIG.15 decreases, and thereby the persistence degree becomes lower.

The insulating layers 24A and 26A are made of, for example, SiNx,SiO2,resist or acrylic resin. In a trial, SiNx was used as the insulatinglayers 24A and 26A, JALS 204 made by JSR Co. as the vertically orientedlayer 27, and ZLI4535 made by Merck Japan Co. as the liquid crystal 30,and the persistence degree reducing effect of the trial article wasconfirmed.

Third Embodiment

FIG. 19 is a plane view showing an electrode pattern of a liquid crystalpixel capable of reducing the persistence degree, of a third embodimentaccording to the present invention, which is analogous to FIG. 33.

The electrode pattern is formed, for example, in the substrate 20A ofFIG. 16 or the substrate 20 of FIG. 31.

FIGS. 20 and 21 are plane views showing the pixel electrode 25A and thecommon electrode 23A of FIG. 19, which are analogous to FIGS. 34 and 35,respectively.

A peripheral section of the pixel or common electrode has crossoverportions to stripe electrodes of the common or pixel electrodes with theinsulating layer interposing therebetween since a pixel has arectangular shape, and each of the pixel electrode 25A and the commonelectrode 23A has stripe electrodes in parallel to each other and has acontinuous shape. For example, a side 251 of the pixel electrode 25A isconnected to a side 252 of the peripheral section, and a side 231 of thecommon electrode 23A is parallel to the side 251, while the sides 252and 232 are connected to the side 251 and 231, respectively, crossesover each other.

FIG. 22(A) is a partial enlarged view near a crossover of electrodes.FIG. 22(B) shows the lines of electric force with dotted lines when avoltage is applied between the pixel electrode 25A and the commonelectrode 23A.

Since the sides 252 and 232 cross over each other at an obtuse angle,concentration of the lines of electric force decreases, and thereby itis suppressed for an electric field strength to become larger incomparison with a case where the sides 252 and 232 cross over each otherat an acute angle as shown in FIG. 36(A).

Further, with respect to a line SA passing through between the sides 251and 231, the sides 251 and 252 are symmetrical to the sides 231 and 232,respectively, resulting in that the direction of electric field vectorbetween the sides 252 and 232 is parallel to that between the sides 251and 231.

Accordingly, rapidly changing distribution of the transmittance nearelectrode crossover is alleviated, with the result that display imagequality is improved and persistence degree is reduced. This holds atother electrode crossovers in a similar way.

Trial liquid crystal panels were fabricated in which the electrodepatterns of FIG. 19 and FIG. 33 were employed both with the otherconditions being the same as those of the above described trial example,and it was confirmed that the liquid crystal panel employing theelectrode pattern of FIG. 19 has a lower persistence degree than thatemploying the electrode pattern of FIG. 33.

Fourth Embodiment

FIG. 23 is a plane view showing an electrode pattern of a liquid crystalpixel capable of reducing the persistence degree, of a fourth embodimentaccording to the present invention, which is analogous to FIG. 19. FIG.24 is a plane view showing the common electrode 23B of FIG. 23, whilethe pixel electrode 25A is the same as that of FIG. 23.

In the peripheral section of the common electrode 23B, cutoff portions23B1 to 23B8 are formed with ensuring one body of the common electrodes23B. The positions of the cutoff portions 23B1 to 23B8 are each nearcrossovers between the common electrode 23B and the pixel electrode 25A.

In a case where non of these cutoff portions exist, an electric fieldarises in a non-display region between these portion and correspondingportions of the pixel electrode 25A when a voltage is applied, whichaffects orientation of liquid crystal molecules in a display region nearthe non-display region. This adverse influence is removed by the cutoffportions, resulting in improving a display image quality and reducingthe persistence degree in comparison with that of the third embodiment.

Fifth Embodiment

FIG. 25 is a plane view showing an electrode pattern of a liquid crystalpixel capable of reducing the persistence degree, of a fifth embodimentaccording to the present invention, which is analogous to FIG. 33. FIG.26 is a plane view showing the common electrode 23C of FIG. 25, whilethe pixel electrode 25 is the same as that of FIG. 34.

In the common electrode 23C, cutoff portions 23B1 to 23B8 are formedwith ensuring one body of the common electrodes 23C, resulting inimproving a display image quality and reducing the persistence degree incomparison with the structure of FIG. 23 for the same reason as that ofthe above described fourth embodiment.

Sixth Embodiment

FIG. 27 is a plane view showing an electrode pattern of two liquidcrystal pixels adjacent to each other, of a sixth embodiment accordingto the present invention, wherein the both pixels have the same pattern.

The frame sections of a common electrode 23D and a pixel electrode 25Doverlap each other with an insulating layer interposing therebetween.The stripe electrode section of the common electrode 23D are formedunder and between stripe electrodes of the pixel electrode 25D, andtherefore the line density of the stripe electrode sections of thecommon electrode 23D is two times greater than that of the pixelelectrode 25D.

FIG. 28 is an enlarged sectional view taken along line A-A of FIG. 27.

Different points from the liquid crystal pixel of FIG. 32 are that thestripe electrodes of the pixel electrode 25D are convex in crosssection, and an insulating layer 26D is formed only on the stripeelectrodes of the pixel electrode 25D and no insulating layer is formedon display areas between stripe electrodes of the common electrode 23Dand the pixel electrode 25D. A vertically oriented layer 27 is thinnerthan the insulating layer 26D, therefore it is depicted with a thickline in FIG. 28.

Since the stripe electrodes of the pixel electrode 25D are convex incross section, the top surface thereof is sloped toward both sides withthe maximum height at the middle. In order to form such a convex shape,unlike FIG. 32, there is formed a stripe electrode of the commonelectrode 23D under each stripe electrode of the pixel electrode 25D,wherein this stripe electrode of the common electrode 23D has a narrowerwidth than that of the pixel electrode 25D. In order to emphasize thisconvex shape, a channel protective layer 41 is partially removed withleaving portions over the stripe electrodes of the common electrode 23D,wherein the channel protective layer 41 is formed when the TFT 29 ofFIG. 27 is formed, and has a width narrower than that of the underlyingstripe electrode.

With such a structure having a convex shape in cross section, the linesof electric force becomes as shown with dotted lines in FIG. 28 when avoltage is applied between the pixel electrode 25D and the commonelectrode 23D. That is, since the lines of electric force near a slopeof the pixel electrode 25D are normal to the slope, inclination ofliquid crystal molecules relative to a normal to the surface of thesubstrate 21 becomes larger, which increases a transmittance in whitedisplay in comparison with the case of FIG. 32, thereby improving adisplay contrast.

Further, since the patterns of the pixel electrode 25D and theinsulating layer 26D are the same as each other, and no insulating layer26D exists in display areas between the stripe electrodes of the pixelelectrode 25D and the common electrode 23D, an electric field is moreeffectively used on liquid crystal molecules in comparison with the caseof FIG. 32, resulting in improving a display contrast in comparison withthe case of FIG. 32 under the same applied voltage.

Furthermore, since liquid crystal molecules do not directly contact withthe pixel electrode 25D, decomposition of liquid crystal molecules areprevented, and the persistence degree is also reduced.

FIGS. 29 and 30 are schematic sectional views showing a fabricationprocess of a substrate 20D on the back light incident side, and eachsection corresponds to one taken along line B-B of FIG. 27. There willbe described below the fabrication process.

(A) A common electrode 23D and a scan (gate) line SLl both made of metalare formed on a substrate 21 by photolithography.

(B) There are formed on the substrate 21 an insulating layer 24, anintrinsic semiconductor layer 42, and a channel protective layer 41. (C)The channel protective layer 41 is partially removed with leavingportions only over the scan line SLl and the common electrode 23D byphotolithography.

(D) An n+semiconductor layer 43, a conductive layer 25D and aninsulating layer 26D are formed on the semiconductor layer 42, and theselayers are etched into the same pattern not only to form the source Sand drain D of TFT 29 over the scan line SL1 but also to simultaneouslyform the stripe electrodes of the pixel electrode 25D and the insulatinglayer 26D over stripe electrodes of the common electrode 23D. Theconductive layer 25D has conductive layers 25 a, 25 b and 25 c, forexample, Ti/Al/Ti. If only the Al layer is used as the electrode 25D,the Al diffuses into the n+semiconductor layer 43, therefore a T1 layeris used in order to avoid this diffusion, while if only the T1 layer isused, a resistance value becomes large, and therefore the Al layer isalso used. The insulating layer 26D is a silicon nitride layer or asilicon oxide layer formed by means of DVD.

Note that if the two layer structure of Ti/Al is used as the pixelelectrode 25D and aluminum nitride is used as the insulating layer 26D,these layers can be continuously grown by a sputter device, resulting inreducing the number of steps of the fabrication process. Further, as theinsulating layer 26D, a photoresist used for patterning may be left overthe stripe electrodes of the common electrode 23D.

(E) A vertically oriented layer 27 drawn with a thick line in FIG. 30 iscoated on the insulating layers 24 and 26.

According to the six embodiment, since by forming the TFT 29, the stripeelectrodes, each having a convex shape in cross section, of the pixelelectrode 25D and the insulating layer 26D thereon are also formedsimultaneously, there is no need to increase the steps of fabricationprocess in order to form the pixel electrode 25D and the insulatinglayer 26D.

Although preferred embodiments of the present invention has beendescribed, it is to be understood that the invention is not limitedthereto and that various changes and modifications may be made withoutdeparting from the spirit and scope of the invention.

1. A liquid crystal panel development method, said liquid crystal panelcomprising: first and second substrate, said first substrate including afirst electrode, said first or second substrate including a secondelectrode, said first and second electrode being for a display voltageto be applied therebetween; and liquid crystal interposed between saidfirst and second substrates; said method comprising the steps of:applying a voltage signal between said first electrode and said secondelectrode, said voltage signal being composed of an AC voltage componentand a DC voltage component Vdc, said AC voltage component having anamplitude Vac; changing said amplitude Vac and said DC voltage componentVdc to measure the range of optimal DC component variation ΔVdc; anddetermining a structure or material of said liquid crystal panel suchthat said range of optimal DC component variation ΔVdc becomes less thana given value; wherein said range of optimal DC component variation ΔVdcis defined as ΔVdc=|Vdcb−Vdcw|, wherein Vdcb is a value of said DCvoltage component Vdc at which a range of transmittance variation is theminimum when said DC voltage component Vdc is changed with saidamplitude Vac being fixed at a value for displaying black, wherein Vdcwis a value of said DC voltage component Vdc at which a range oftransmittance variation is the minimum when said DC voltage componentVdc is changed with said amplitude Vac being fixed at a value fordisplaying white.
 2. The liquid crystal panel development method ofclaim 1, wherein said given value is equal to or less than 0.5 V.